1. Field of the Invention
The present invention relates to a display device comprising light emitting elements and to a driving method thereof.
2. Description of the Related Art
In recent years, a display device using light emitting elements (self light emitting elements) has been studied and developed. Such a display device has the advantages of high image quality, reduced thickness, light weight, and the like, and thus it is widely used as a display screen of a mobile phone and a monitor of a personal computer. In particular, the light emitting display device has the features such as low voltage driving, low power consumption, and fast response which contributes to dynamic display, therefore, it is expected to be widely used for various electronic apparatuses typified by a next-generation mobile phone or portable information terminal (PDA).
A light emitting element comprises an anode, a cathode, and a layer including an electro luminescent material (hereinafter referred to as an electro luminescent layer) which generates the electro luminescence when an electric field is applied. The electro luminescent layer is sandwiched between the anode and the cathode. There is a fixed relation between the amount of current flowing in the light emitting element and the luminance thereof, and the light emitting element emits light at a luminance corresponding to the amount of current flowing in its electro luminescent layer.
For displaying a multi-level image in the light emitting display device, either an analog driving method (analog gray scale display) or a digital driving method (digital gray scale display) is adopted. These methods are different in the way of controlling a light emitting element during a light emission or a non-light emission of the light emitting element.
In the analog driving method, gray scale is achieved by continuously controlling the amount of current flowing in the light emitting element. Meanwhile, in the digital driving method, the display device is driven by switching two states of the light emitting element, namely, ON (the luminance is substantially 100%) and OFF (the luminance is substantially 0%) states.
Brief explanation is hereinafter made on a pixel configuration of a display device using a common digital driving method as well as an operation of the pixel. A pixel shown in FIG. 8 comprises a switching transistor 700, a driving transistor 701, a capacitor 702, and a light emitting element 703. A gate of the switching transistor 700 is connected to a scan line 705, either a source or a drain thereof is connected to a signal line 704, and the other is connected to a gate of the driving transistor 701. A source of the driving transistor 701 is connected to a power supply line 706 and a drain thereof is connected to an anode of the light emitting element 703. A cathode of the light emitting element 703 is connected to a counter electrode 707. The capacitor 702 is provided in order to hold a potential difference between the gate and the source of the driving transistor 701. A predetermined voltage is applied from a power supply to each of the power supply line 706 and the counter electrode 707 so as to have a potential difference therebetween.
When the switching transistor 700 is turned ON by a signal from the scan line 705, a video signal is inputted from the signal line 704 to the gate of the driving transistor 701. A potential difference between the inputted video signal and the power supply line 706 corresponds to a gate-source voltage Vgs of the driving transistor 701, and thus a current is supplied to the light emitting element 703 which emits light consequently.
In the digital driving method, however, not more than two-level gray scale display can be achieved without any complement. Therefore, area gray scale display or time gray scale display is suggested to be used as a driving method for displaying a multi-level image. The area gray scale display is a method in which a pixel is divided into sub pixels so as to have different light emitting areas and gray scale display is achieved by selecting the sub pixels. The area gray scale display is specifically disclosed in Patent Document 1.
[Patent Document 1]
Japanese Patent Laid-Open No. H11-73158